The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example. U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These hydrogel-forming absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble yet water-swellable, by slightly cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N,N'-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
These hydrogel-forming absorbent polymers are also often made by initially polymerizing unsaturated amines or derivatives thereof such as diallyldimethylammonium chloride, N,N-dimethylaminoethylmethacrylate.HCl, N,N-dimethylaminoethylacrylate.HCl, methacrylamido-propyltrimethyl-ammonium hydroxide and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly cross-linking the polymer chains with conventional di- or poly-functional monomer materials, such as N,N'-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of cationic (charged) amine groups attached to the polymer backbone. It is these charged amine groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
The degree of cross-linking determines not only the water-insolubility of these hydrogel-forming absorbent polymers, but is also an important factor in establishing two other characteristics of these polymers: their absorbent capacity and gel strength. Absorbent capacity or "gel volume" is a measure of the amount of water or body fluid that a given amount of hydrogel-forming polymer will absorb. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or "flow" under an applied stress. Hydrogel-forming polymers useful as absorbents in absorbent structures and articles such as disposable diapers need to have adequately high gel volume, as well as adequately high gel strength. Gel volume needs to be sufficiently high to enable the hydrogel-forming polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution throughout the structure/article. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al). issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
These hydrogel-forming polymers are typically lightly-crosslinked polyelectrolytes that swell in aqueous electrolyte solutions primarily as a result of an osmotic driving force. The osmotic driving force for hydrogel-forming polymer swelling results primarily from polyelectrolyte counterions that are dissociated from the polyelectrolyte but are kept inside the swollen polymer due to electroneutrality considerations. Hydrogel-forming polymers that comprise weak-acid or weak-base polyelectrolytes (e.g., carboxylic acid or mono/di/tri- amine functional groups) in their un-neutralized forms are only slightly dissociated in urine solutions. These weak-acid or weak-base hydrogel-forming polymers must be at least partially neutralized with base or acid, respectively, in order to generate substantial concentrations of dissociated counterions. Without neutralization to e.g., .about.70%, these weak-acid or weak-base hydrogel-forming polymers do not swell to their maximum potential absorbent capacity or gel volume. In contrast, the absorbent capacity of hydrogel-forming polymers comprising strong-acid or strong-base functional groups (e.g., sulfonic acid or quaternary ammonium hydroxide) are much less sensitive to their degree of neutralization. However, the use of these strong-acid or strong-base hydrogel-forming polymers in their un-neutralized forms have the potential to shift the pH of the urine solution to unacceptably low or high values, respectively.
Even after neutralization, the osmotic driving force for swelling and thus the absorbent capacity or gel volume of polyelectrolyte hydrogel-forming polymers is greatly depressed by the high concentration of dissolved electrolyte normally present in urine. The concentration of this dissolved electrolyte, expressed as wt % NaCl, can be as high as 0.9% (physiological saline) or higher. It is known that reducing the concentration of dissolved electrolyte in urine (e.g., by dilution with distilled water) can greatly increase the absorbent capacity of a polyelectrolyte hydrogel-forming polymer. Thus, for example, when Jayco synthetic urine is used to measure the gel volume of a partially-neutralized sodium polyacrylate hydrogel-forming polymer, a ten-fold dilution of Jayco with distilled water can results in approximately a three-fold increase in gel volume.
It is known that the concentration of dissolved electrolyte in an aqueous solutions can be lowered by "reaction" of the solution with a mixed-bed ion-exchange resin. (Ion-exchange columns are often used commercially to deionize water.) Electrolyte concentration is reduced by the combined effect of (i) exchange of dissolved cations (e.g., Na.sup.+) in the aqueous solution with H.sup.+ from the cation-exchange resin and (ii) exchange of dissolved anions (e.g., Cl.sup.-) with OH.sup.- from the anion-exchange resin. The H.sup.+ and OH.sup.- from the resins combine in solution to yield H.sub.2 O. It is the reaction of H.sup.+ and OH.sup.- to form H.sub.2 O that drives the transfer of dissolved anions and cations from solution onto their respective resins, resulting in a reduction in solution electrolyte concentration. Generally, mixed-bed resins contain approximately equal equivalents of anion-exchange and cation-exchange functional groups. Particles of anion and cation resins are desirably intimately mixed and/or have high surface areas in order to shorten diffusion distances and increase ion-exchange rates.
Ion-exchange resins have been used to increase the absorbent capacity of absorbent articles containing hydrogel-forming polymers. See, for example, U.S. Pat. No. 4,818,598 issued Apr. 4, 1989 to Wong. However, the need to incorporate large quantities of ion-exchange resin(s) have little or no absorbent capacity generally increases the bulk of the absorbent article to an unacceptable degree.
It is known that a mixture of (i) an anionic hydrogel-forming polymer in its acid form and (ii) a cationic hydrogel-forming polymer in its base form has the potential to function as a mixed-bed ion-exchange system with respect to the reduction of solution electrolyte concentration. Furthermore, if the anionic hydrogel-forming polymer in a mixed-bed ion-exchange system is a weak acid and starts off in its un-neutralized form, then the resulting exchange of H.sup.+ by e.g., Na.sup.+ results in the conversion of the anionic hydrogel-forming polymer from its un-neutralized to neutralized form. Thus, the osmotic driving force for swelling (and thus the absorption capacity of the hydrogel-forming polymer) of a weak-acid anionic hydrogel-forming polymer increases as a result of ion-exchange in a mixed-bed ion-exchange system. Similarly, if the cationic hydrogel-forming polymer in a mixed-bed ion-exchange system is a weak base and starts off in its un-neutralized form, then the resulting exchange of OH.sup.- by e.g., Cl.sup.- (or the addition of HCl to a neutral amine group) results in the conversion of the cationic hydrogel-forming polymer from its un-neutralized to neutralized form. Thus, the osmotic driving force for swelling of a weak-base cationic hydrogel-forming polymer also increases as a result of ion-exchange in a mixed-bed ion-exchange system. Whether or not the hydrogel-forming polymers in a mixed-bed ion-exchange system are weak/strong acids or weak/strong bases, the reaction of an aqueous electrolyte solution with a mixed-bed ion-exchange system results in at least some lowering of electrolyte concentration, which results in at least some increase in the osmotic driving force for swelling. As a result of the combined effects of (i) reduction in electrolyte concentration and (ii) conversion (if necessary) from a less-swellable to a more-swellable form, a mixed-bed ion-exchange hydrogel-forming polymer system, where the anionic and cationic hydrogel-forming polymers each start out in their un-neutralized forms, has the potential to deliver an increased osmotic driving force for swelling relative to a mixture of comparable anionic and cationic hydrogel-forming polymers where they each start off in their neutralized forms. The use of mixed-bed ion-exchange hydrogel-forming polymers to increase absorption capacity has been described in PCT Applications WO 96/17681 (Palumbo; published Jun. 13, 1996), WO 96/15162 (Fornasari et. al., published May 23, 1996), and U.S. Pat. No. 5,274,018 (Tanaka; issued Dec. 28, 1993).
The degree to which a mixed-bed ion-exchange hydrogel-forming polymer system can potentially reduce electrolyte concentration depends on (i) the meq/g ion-exchange capacity of the anionic and cationic hydrogel-forming polymers; (ii) the pKa and pKb (and thus the extent of reaction) of the anionic and cationic hydrogel-forming polymers; (iii) meq/l of electrolyte in the aqueous solution; and (iv) the l/g ratio of aqueous electrolyte solution to ion-exchange hydrogel-forming polymers. For a given mixed-bed ion-exchange capacity, pKa and pKb, and electrolyte concentration, the reduction in electrolyte concentration is maximized by minimizing the total volume of solution in contact with the ion-exchange hydrogel-forming polymers. In an absorbent structure (e.g., a blend of hydrogel-forming polymers and fiber), only a portion of the total fluid is absorbed by the hydrogel-forming polymer. The balance of the fluid is absorbed by other components (e.g., in pores formed by the fiber structure). However, even though this fluid is not absorbed by the hydrogel-forming polymer, the electrolyte in this fluid can diffuse into the hydrogel-forming polymer and thus raise the electrolyte concentration within to a level greater than if the external fluid was not present. If the objective is to use a mixed-bed ion-exchange hydrogel-forming polymer system to increase absorbent capacity, then the potential benefits of ion-exchange are lessened as the percentage of hydrogel-forming polymer in the absorbent structure is decreased. In contrast, reducing the quantity of fiber (or other non-hydrogel-forming-polymer components capable of absorbing fluid) minimizes the quantity of extra solution and thus the quantity of extra salt that must be exchanged in order to achieve a given reduction in electrolyte concentration. Thus, in principle, when a mixed-bed ion-exchange hydrogel-forming-polymer system is incorporated in an absorbent structure, it can benefit to a greater degree from ion-exchange when it is incorporated at high concentration versus at low concentration.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Using lower concentrations of these hydrogel-forming absorbent polymers limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume.
Besides increasing gel strength, other physical and chemical characteristics of these hydrogel-forming absorbent polymers have been manipulated to decrease gel blocking. One characteristic is the particle size, and especially the particle size distribution, of the hydrogel-forming absorbent polymer used in the fibrous matrix. For example, particles of hydrogel-forming absorbent polymer having a particle size distribution such that the particles have a mass median particle size greater than or equal to about 400 microns have been mixed with hydrophilic fibrous materials to minimize gel blocking and to help maintain an open capillary structure within the absorbent structure so as to enhance planar transport of fluids away from the area of initial discharge to the rest of the absorbent structure. In addition, the particle size distribution of the hydrogel-forming absorbent polymer can be controlled to improve absorbent capacity and efficiency of the particles employed in the absorbent structure. See U.S. Pat. No. 5,047,023 (Berg), issued Sep. 10, 1991. However, even adjusting the particle size distribution does not, by itself, lead to absorbent structures that can have relatively high concentrations of these hydrogel-forming absorbent polymers. See U.S. Pat. No. 5,047,023, supra (optimum fiber to particle ratio on cost/performance basis is from about 75:25 to about 90:10).
Another characteristic of these hydrogel-forming absorbent polymers that has been looked at is the level of extractables present in the polymer itself. See U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. No. 32,649). Many of these hydrogel-forming absorbent polymers contain significant levels of extractable polymer material. This extractable polymer material can be leached out from the resultant hydrogel by body fluids (e.g., urine) during the time period such body fluids remain in contact with the hydrogel-forming absorbent polymer. It is believed such polymer material extracted by body fluid in this manner can alter both the chemical and physical characteristics of the body fluid to the extent that the fluid is more slowly absorbed and more poorly held by the hydrogel in the absorbent article.
Another characteristic that has been looked at to minimize gel blocking is to improve the capillary capability of these hydrogel-forming absorbent polymers. In particular, it has been suggested that particles of these hydrogel-forming absorbent polymers be formed into interparticle crosslinked aggregate macrostructures, typically in the form of sheets or strips. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992; U.S. Pat. No. 5,124,188 (Roe et al), issued Jun. 23, 1992; and U.S. Pat. No. 5,149,344 (Lahrman et al), issued Sep. 22, 1992. Because the particulate nature of the absorbent polymer is retained, these macrostructures provide pores between adjacent particles that are interconnected such that the macrostructure is fluid permeable (i.e., has capillary transport channels). Due to the interparticle crosslink bonds formed between the particles, the resultant macrostructures also have improved structural integrity, increased fluid acquisition and distribution rates, and minimal gel blocking characteristics.
Yet another characteristic the art has known for some time as a measure of gel blocking is the Demand Wettability or Gravimetric Absorbence of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 5,562,646 (Goldman et. al.) issued Oct. 8, 1996 and U.S. Pat. No. 5,599,335 (Goldman et. al.) issued Feb. 4, 1997 where Demand Wettability/Gravimetric Absorbence is referred to as Performance Under Pressure (PUP). In a PUP experiment, an initially-dry AGM at 100% concentration is positioned in a piston/cylinder apparatus (where the bottom of the cylinder is permeable to solution, but impermeable to the AGM) under a mechanical confining pressure and is allowed to absorb synthetic urine under demand-absorbency conditions at zero hydrostatic suction and high mechanical pressure. The "PUP" capacity is defined as the g/g absorption of Jayco Synthetic Urine by a 0.032 g/cm2 layer of the hydrogel-forming absorbent polymer, while being confined under an applied pressure of 5 KPa (about 0.7 psi) for a time period of one hour. A hydrogel-forming polymer is deemed to have desirable PUP properties if it absorbs at least about 23 g/g after one hour. A high PUP capacity is a critically important property for a hydrogel-forming polymer when it is used at high concentrations in an absorbent structure.
Although maximizing the concentration of mixed-bed ion-exchange hydrogel-forming-polymers an absorbent structure increases the osmotic driving force for swelling, this increase in osmotic driving force has heretofore not resulted in the anticipated improvement in absorbency performance in terms of PUP capacity. It is believed that the performance deficiency at high concentration of current mixed-bed ion-exchange hydrogel-forming polymers results at least in-part from the constituent polymers in the mixed-bed system and their mixture not being optimized for use at high concentration and high confining pressure. As a result, current mixed-bed ion-exchange hydrogel-forming polymers tend to gel block under a confining pressure, exhibit slow absorption rates under PUP-absorption conditions, and have a low PUP absorption capacity after a reasonable period of time. As a result, the PUP absorption of the mixed-bed ion-exchange hydrogel-forming polymers is not significantly greater than the PUP absorption of a comparable mixture of the cationic and anionic hydrogel-forming polymers, where the polymers are neutralized prior to the PUP measurement or of either the anionic or cationic hydrogel-forming polymer by itself, where the polymers are neutralized prior to the PUP measurement. (They can also exhibit a low value for saline flow conductivity (SFC), a low value for Porosity of the Hydrogel Layer (PHL), and slow ion-exchange rates--see discussion below.) The deficiencies of current mixed bed ion-exchange hydrogel-forming polymers at high concentrations is especially noteworthy, given the importance of using hydrogel-forming polymers in high concentrations in absorbent articles such as diapers.
For absorbent structures having relatively high concentrations of these hydrogel-forming absorbent polymers, other characteristics of these absorbent polymers are also important. It has been found that the openness or porosity of the hydrogel layer formed when these absorbent polymers swell in the presence of body fluids is relevant for determining the ability of these absorbent polymers to acquire and transport fluids, especially when the absorbent polymer is present at high concentrations in the absorbent structure. Porosity refers to the fractional volume that is not occupied by solid material. For a hydrogel layer formed entirely from a hydrogel-forming absorbent polymer, porosity is the fractional volume of the layer that is not occupied by hydrogel. For an absorbent structure containing the hydrogel, as well as other components, porosity is the fractional volume (also referred to as void volume) that is not occupied by the hydrogel, or other solid components (e.g., fibers).
The openness or porosity of a hydrogel layer formed from a hydrogel-forming absorbent polymer can be defined in terms of Porosity of the Hydrogel Layer (see, for example, U.S. Pat. No. 5,562,646). A good example of a material having a very-high degree openness is an air-laid web of wood-pulp fibers. For example, the fractional degree of openness of an air-laid web of wood pulp fibers (e.g., having a density of 0.15 g/cc) is estimated to be 0.8-0.9, when wetted with body fluids under a confining pressure of 0.3 psi.
It has been found that the PHL value of the hydrogel-forming absorbent polymer does not have to approach that of an air-laid web of wood pulp fibers in order to obtain substantial performance benefits when these absorbent polymers are present at high concentrations. These benefits include (1) increased void volume in the resultant hydrogel layer for acquiring and distributing fluid; and (2) increased total quantity of fluid absorbed by the absorbent polymer under demand wettability/gravimetric absorbency conditions (i.e., for the storage of fluid). Increased porosity can also provide additional performance benefits such as: (3) increased permeability of the resultant hydrogel layer for acquiring and distributing fluid; (4) improved wicking properties for the resultant hydrogel layer, such as wicking fluid upwardly against gravitational pressures or partitioning fluid away from an acquisition layer; and (5) improved swelling-rate properties for the resultant hydrogel layer to allow more-rapid storage of fluid. A hydrogel-forming polymer is deemed to have desirable PHL properties if its PHL value is at least about 0.15.
Another important property at higher concentrations of these hydrogel-forming absorbent polymers is their permeability/flow conductivity. Permeability/flow conductivity can be defined in terms of their Saline Flow Conductivity (SFC) values. SFC measures the ability of a material to transport saline fluids such as the ability of the hydrogel layer formed from the swollen hydrogel-forming absorbent polymer to transport body fluids. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) will exhibit an SFC value of about 200.times.10.sup.-7 cm.sup.3 sec/g. Accordingly, it would be highly desirable to be able to use hydrogel-forming absorbent polymers that more closely approach an air-laid web of wood pulp fibers in terms of SFC. A hydrogel-forming polymer is deemed to have desirable permeability properties if its SFC value is at least about 30.times.10.sup.-7 cm.sup.3 sec/g.
Another factor that has to be considered in order to take full advantage of the porosity and permeability properties of the hydrogel layer formed from these absorbent polymers is the wet integrity of the region or regions in the absorbent member that comprise these polymers. For hydrogel-forming absorbent polymers having relatively high porosity and SFC values, it is important that the region(s) in which polymers are present have good wet integrity. By "good wet integrity" is meant that the region or regions in the absorbent member having the high concentration of hydrogel-forming absorbent polymer have sufficient integrity in a dry, partially wet, and/or wetted state such that the physical continuity (and thus the capability of acquiring and transporting fluid into and through contiguous interstitial voids/capillaries) of the hydrogel formed upon swelling in the presence of body fluids is not substantially disrupted or altered, even when subjected to normal use conditions. During normal use, absorbent cores in absorbent articles are typically subjected to tensional and torsional forces of varying intensity and direction. These tensional and torsional forces include bunching in the crotch area, stretching and twisting forces as the person wearing the amount article walks, squats, bends, and the like. If wet integrity is inadequate, these tensional anti torsional forces can potentially cause a substantial alteration and/or disruption in the physical continuity of the hydrogel such that its capability of acquiring and transporting fluids into and through the contiguous voids and capillaries is degraded, e.g., the hydrogel laser can be partially separated, fully separated, have gaps introduced, have areas that are significantly thinned, and/or broken up into a plurality of significantly smaller segments. Such alteration could minimize or completely negate any advantageous porosity and permeability/flow conductivity properties of the hydrogel-forming absorbent polymer.
Accordingly, it would be desirable to be able to provide mixed-bed ion-exchange hydrogel-forming polymers capable of absorbing an increased quantity of an urine electrolyte solution under PUP-absorption conditions in a reasonable period of time relative to a comparable mixture of the constituent hydrogel-forming polymers, each in their neutralized forms. It would also be desirable to be able to provide mixed-bed ion-exchange hydrogel-forming polymers capable of absorbing a large quantity of an urine electrolyte solution under PUP-absorption conditions in a reasonable period of time It would also be desirable to provide an absorbent structures containing a high concentration of a mixed-bed ion-exchange hydrogel-forming polymer capable of absorbing an increased quantity of an urine electrolyte solution under PUP-absorption conditions in a reasonable period of time relative to a comparable mixture of the constituent hydrogel-forming polymers, each in their neutralized forms. It would also be desirable to provide a mixed-bed ion-exchange hydrogel-forming polymer having high SFC and PHL values.